1. Field of the Invention
The present invention relates to a method and apparatus for channel availability query on multiple locations in a wireless local area network (WLAN).
2. Discussion of the Related Art
Standards for wireless local area network (WLAN) technologies have been developed as IEEE 802.11 standards. IEEE 802.11a provides a transmission rate of 54 Mbps and IEEE 802.11b provides a transmission rate of 11 Mbps using an unlicensed band at 2.4 GHz or 5 GHz. IEEE 802.11g provides a transmission rate of 54 Mbps using orthogonal frequency-division multiplexing (OFDM) at 2.4 GHz. IEEE 802.11n provides a transmission rate of 300 Mbps for 4 spatial streams using Multiple Input Multiple Output OFDM (MIMO-OFDM). IEEE 802.11n supports channel bandwidths of up to 40 MHz and, in this case, provides a transmission rate of 600 Mbps.
IEEE 802.11af standards for regulating WLAN operation of an unlicensed device in a TV whitespace zone is under development.
TV whitespace includes VHF bands (54 to 60 MHz, 76 to 88 MHz, and 174 to 216 MHz) and UHF bands (470 to 698 MHz) which are assigned to broadcast TV. The TV whitespace means a frequency band allowed to be used by an unlicensed device on the condition that the unlicensed device does not disturb communication of licensed devices (TV broadcast, wireless microphone, etc.) operating in the frequency band.
While all unlicensed devices are permitted to operate in frequency bands of 512 to 608 MHz and 614 to 698 MHz except some special cases, frequency bands of 54 to 60 MHz, 76 to 88 MHz, 174 to 216 MHz and 470 to 512 MHz are allowed only for communication between fixed devices. A fixed device means a device that transmits signals only at a fixed location. In the following description, a whitespace band includes the aforementioned TV whitespace. However, the whitespace is not limited thereto.
An unlicensed device that wants to use a whitespace band needs to provide a protection function for licensed devices. Accordingly, the unlicensed device must check whether the licensed devices occupy the whitespace band before starting transmission in the whitespace band.
To achieve this, the unlicensed device needs to access a geo-location database through the Internet or a dedicated network to acquire information on a list of channels available in the corresponding area. The geo-location database stores and manages information on licensed devices registered therein and channel usage information that is dynamically changed according to geo-locations and used time of the licensed devices.
A station (STA) can execute a spectrum sensing mechanism. An energy detection scheme, a feature detection scheme, etc. can be used as the spectrum sensing mechanism. For example, it is possible to determine that a channel is being used by a licensed device or an incumbent user when the intensity of a received signal exceeds a predetermined value or a DTV preamble is detected. When it is determined that a channel adjacent to a currently used channel is used by an incumbent user, an STA and an AP need to decrease transmit power.